The present invention relates to a composition comprising a quasi-crystalline hydrated magnesium-aluminium hydroxy carboxylate, its preparation and use in catalyst compositions. The invention also relates to the preparation of magnesium-aluminium solid solutions and anionic clays by using the carboxylate compositions as intermediate.
A variety of terms are used to describe the material that is referred to in this specification as an anionic clay. Hydrotalcite-like and layered double hydroxide is interchangeably used by those skilled in the art. In this specification we refer to these materials as anionic clays, comprising within that term hydrotalcite-like and layered double hydroxide materials.
Anionic clays have many applications. These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical field. In particular Van Broekhoven (U.S. Pat. Nos. 4,956,581 and 4,952,382) has described their use in SOx abatement chemistry.
The preparation of anionic clays has been described in many prior art publications. Articles relating to anionic clays include:
Helv. Chim. Acta, 25, 106-137 and 555-569 (1942)
J. Am. Ceram. Soc., 42, no. 3, 121 (1959)
Chemistry Letters (Japan), 843 (1973)
Clays and Clay Minerals, 23, 369 (1975)
Clays and Clay Minerals, 28, 50 (1980)
Clays and Clay Minerals, 34, 507 (1996)
Materials Chemistry and Physics, 14, 569 (1986).
There is also extensive patent literature on the use of anionic clays and processes for their preparation.
Two major reviews of anionic clay chemistry were published in which the synthesis methods available for anionic clay synthesis have been summarised: F. Cavani et al xe2x80x9cHydrotalcite-type anionic clays: Preparation, Properties and Applications,xe2x80x9d Catalysis Todayxe2x80x9d, 11 (1991) Elsevier Science Publishers B. V. Amsterdam; and J P Besse and others xe2x80x9cAnionic clays: trends in pillary chemistry, its synthesis and microporous solidsxe2x80x9d(1992), 2, 108, editors: M. I. Occelli, H. E. Robson, Van Nostrand Reinhold, N.Y.
Two types of anionic clay preparation are described in these reviews. The most conventional method is co-precipitation (in Besse this method is called the salt-base method) of a soluble divalent metal salt and a soluble trivalent metal salt under alkaline conditions, optionally followed by hydrothermal treatment or aging to increase the crystallite size. The second method is the salt-oxide method in which a divalent metal oxide is reacted at atmospheric pressure with a soluble trivalent metal salt, followed by aging under atmospheric pressure. This method has only been described for the use of ZnO and CuO in combination with soluble trivalent metal salts.
The prior art anionic clays are all prepared by reaction of a magnesium source and an aluminium source under basic conditions, most typically at pH values in the range 8-10 and above. The basic reaction environment, however, leads to corrosion of equipment and limits the processing conditions of their preparation and any subsequent reaction. It is therefore an object of this invention to prepare anionic clays in mildly acidic environment.
In the above mentioned reviews the authors state that a characteristic of anionic clays is that mild calcination at 500xc2x0 C. results in the formation of a disordered MgO-like product. Said disordered MgO-like product is distinguishable from spinel (which results upon severe calcination) and from anionic clays. In this specification we refer to said disordered MgO-like materials as Mgxe2x80x94Al solid solutions. In contrast to spinel, which is a stable, irreversible phase, these Mgxe2x80x94Al solid solutions contain a well-known memory effect whereby the exposure to water of such calcined materials results in the reformation of the anionic clay structure. These solid solutions are the active SOx adsorbers under FCC regenerator conditions. Like anionic clays, prior art Mgxe2x80x94Al solid solutions are prepared under basic conditions.
The production of spinel at acidic conditions is disclosed in EP 0 573 610. The disclosed process consists of rapidly drying an acidic slurry of a magnesium and an aluminium compound, followed by calcination.
A further object of the present invention is a new process for the production of anionic clays.
In one embodiment, the present invention comprises new compositions of matter, referred to as quasi-crystalline carboxylates (QCCs), which are quasi-crystalline hydrated magnesium-aluminium hydroxy carboxylates. These quasi-crystalline carboxylates are characterised at least by a strong reflection in the powder X-ray diffraction pattern at a basal spacing in the range of 5 to 15 xc3x85. The QCCs according to the invention may optionally comprise a hydrated magnesium hydroxy carboxylate, a hydrated aluminium hydroxy carboxylate and/or aluminium oxide.
In another embodiment, the invention comprises a process for preparing these QCCs, a process to prepare from them Mgxe2x80x94Al solid solutions, and a process to prepare from the latter anionic clays. The process to prepare QCCs comprises aging of an acidic mixture comprising a magnesium carboxylate and an aluminium source. Calcination of the QCC results in a Mgxe2x80x94Al solid solution, and rehydration of the solid solution gives an anionic clay.
Other embodiments of the invention relate to use of the QCCs of the invention, such as in a catalyst composition.